This invention relates to polymeric alkoxysilanes which are useful as binders for investment castings and zinc-rich paints, and to a process for making such polymeric alkoxysilanes.
Various organopolysiloxanes have been described in the prior art. These materials are generally prepared by partial hydrolysis of silicates or orthosilicates, and function as binders by curing in the presence of atmospheric moisture. However these products are subject to several disadvantages when used in commercial applications. For example, the presence of residual hydroxyl functionality in the polymer causes premature gellation even in the absence of moisture. Thus, the products must be sold with gel-time specifications, indicating the shelflife of the product, which is generally about one year.
Attempts have been made to introduce various organic moieties into these materials. Typical are the polymercaptoorgano and polyhydroxyorgano silanes and siloxanes described in U.S. Pat. No. 3,388,144 to M. C. Musolf et al. and the organopolysiloxanes having terminal acyloxy groups disclosed in U.S. Pat. No. 3,595,885 to G. Rossmy et al. However, the resulting siloxanes are not solvent-resistant, limiting their applications to areas where this property is not required.
Other efforts have been directed to preparing organo polysiloxanes which are stable under certain conditions but have limited applications. For example, in U.S. Pat. No. 3,804,639 to Trulsson et al. it was proposed that a tetraalkyl or tetraalkoxyalkyl orthosilicate be condensed in a hydroxylic solvent, e.g., a lower alkanol or a monoalkyl ether of a glycol in the presence of a peroxide and a catalytic amount of a suitable strong organic acid, e.g., p-toluenesulfonic acid. The reaction caused the exchange of the alkoxy groups of the hydroxylic solvent for one or more of the alkoxy groups of the orthosilicate reagents. The ultimate product could be cured in the complete absence of moisture. However, these compositions, when cured, do not have a sufficient SiO.sub.2 content to render them useful for investment casting where a high inorganic content is essential. Furthermore, the use of organic solvents and peroxides presents safety problems, in the formulation, storage and use of the resultant binder.
Still another approach described in the art involved introducing sulfate groups into organosiloxanes. For example, polysiloxane mixtures with terminal sulfuric acid groups are described in U.S. Pat. No. 3,655,712 to G. Rossmy. The mixtures are prepared by reacting certain organopolysiloxanes with H.sub.2 S.sub.2 O.sub.7 or sulfuric acid, or by reacting organo-halopolysiloxanes with sulfuric acid. The presence of the terminal sulfuric acid groups does not stabilize the mixture towards premature gellation, since hydroxyl groups are present. Furthermore, the mixtures are derived from organosilicones making them unsuitable for casting applications where the solubility of the binding resin is important.
Another example of sulfonated organosilicon compounds is described in U.S. Pat. No. 3,187,033 to S. Nitzsche et al. The silane and organosiloxane starting materials are sulfonated to carbon atoms and not silicon atoms, resulting in materials that are hydrolytically stable and impervious to curing.